Kofron et al U.S. Pat. No. 4,439,520 ushered in the current era of high speed, high performance silver halide photography. Kofron et al disclosed and demonstrated striking photographic advantages for chemically and spectrally sensitized tabular grain emulsions in which tabular grains having a diameter of at least 0.6 .mu.m and a thickness of less than 0.3 .mu.m exhibit an average aspect ratio of greater than 8 and account for greater than 50 percent of total grain projected area. In the numerous emulsions demonstrated one or more of these numerical parameters often far exceeded the stated requirements. Kofron et al recognized that the chemically and spectrally sensitized emulsions disclosed in one or more of their various forms would be useful in color photography and in black-and-white photography (including indirect radiography). Spectral sensitizations in all portions of the visible spectrum and at longer wavelengths were addressed as well as orthochromatic and panchromatic spectral sensitizations for black-and-white imaging applications. Kofron et al employed combinations of one or more spectral sensitizing dyes along with middle chalcogen (e.g., sulfur) and/or noble metal (e.g., gold) chemical sensitizations, although still other, conventional sensitizations, such as reduction sensitization were also disclosed.
An early, cross-referenced variation on the teachings of Kofron et al was provided by Maskasky U.S. Pat. No. 4,435,501, hereinafter referred to as Maskasky I. Maskasky I recognized that a site director, such as iodide ion, an aminoazaindene, or a selected spectral sensitizing dye, adsorbed to the surfaces of host tabular grains was capable of directing silver salt epitaxy to selected sites, typically the edges and/or corners, of the host grains. Depending upon the composition and site of the silver salt epitaxy, significant increases in speed were observed. The most highly controlled site depositions (e.g., corner specific epitaxy siting) and the highest photographic speeds reported by Maskasky I were obtained by epitaxially depositing silver chloride onto silver iodobromide tabular grains. Maskasky I taught a preference for epitaxially depositing a silver salt having a higher solubility than the host tabular grains, stating that this reduces any tendency toward dissolution of the tabular grains while silver salt is being deposited. Maskasky I recognized that even when chloride is the sole halide run into a tabular grain emulsion during epitaxial deposition, a minor portion of the halide contained in the host tabular grains can migrate to the silver chloride epitaxy. However, the concentration in the epitaxy of any halide derived solely from the host tabular grain cannot be higher in the epitaxy than it is in the adjacent portion of the host tabular grain.
While Kofron et al and Maskasky I both acknowledged the possibility of very large average tabular grain sizes, ranging up to 10, 20 or even 30 micrometers (.mu.m), in fact, no Examples of emulsions having mean grain sizes as large as 10 .mu.m were reported by Kofron et al or Maskasky I, and for the most part their Examples of tabular grain emulsions exhibited mean ECD's of .ltoreq.3.0 .mu.m. This preference for tabular grain emulsions having mean ECD's of .ltoreq.3.0 .mu.m has continued up until this invention. Further, the art has recognized that the photographic speeds of tabular grain emulsions reach a maximum at a mean ECD of approximately 5 .mu.m and cannot be increased by further increasing grain size, resulting instead in only increases in granularity.
One of the primary deterrents to employing large (.gtoreq.3.5 .mu.m) mean ECD tabular grain emulsions is based on the disadvantage that image contrast declines progressively as mean ECD's are increased.